It is hard for western politicians not to sound interchangeable in the shadow of an event like the killing of Benazir Bhutto. Almost inevitably, the slaying of the Pakistani opposition leader has triggered a chorus of condemnation from western leaders. Most of the responses have drawn on similar themes, predictably emphasising the principles that western democracies have in common - elected governments, freedoms, the rule of law, orderly political transitions - and pledging to help support them in Pakistan.

In the face of what was surely a reactionary Islamist attack on a Muslim moderniser, it is a natural reflex to assert these values. Yet the cultural differences between democracies can matter almost as much as their similarities. Failure to acknowledge these differences may lure the democracies - and their opponents - into assuming they do not matter much. Yet the history of the early years of the 21st century shows they can be crucial.

In the coming weeks and months, world political attention will shift towards the early phases of the 2008 US presidential election. Next week's Iowa caucuses, followed by the New Hampshire primary on January 8, will begin to define the political choices for the post-George Bush era. Thirteen months from now, the next president will take office amid considerable international goodwill and expectation after the disaster of the Bush years. Yet, unless we understand the dynamics of the process that kicks off next week, that goodwill and expectation may soon seem to have been horribly misplaced.

This is in large part because, while America often talks the same political language as the rest of us, the dynamics of its domestic politics are increasingly discordant with those of other developed democracies. US party politics has become so polarised that the country's ability to solve its own domestic problems, much less those of the world, has been hugely compromised.

The Bill Clinton and George Bush years enabled most overseas observers to grasp that the US is a very foreign country. Why US domestic politics became so polarised at a time when the politics of most European countries became much more consensual is complex. But it is the case, and the process has deepened as America's "baby boomers" - the Vietnam war generation - have become the republic's legislators and rulers.

The result is a political culture preoccupied with satisfying the demands of the party "base" rather than achieving a national consensus. Under Bush, the Republicans became a warrior party that aimed to deliver to its own voters with as little concession to others as possible. In response, especially since Iraq, many Democrats have demanded their party reply in kind. The tendency to fanaticism in the blogosphere reflects and encourages the process. The shape of the upcoming primary races in both parties embodies these tensions and pressures.

The upshot is that while other democratic traditions have tended to become more pragmatic and consensual, those of the US have gone in the other direction. Uniquely, it has become much more ideological, embittered and confrontational at the same time as others become more centrist. Disraeli's phrase about a Britain of two nations, between which there is "no intercourse and no sympathy", seems to fit the new world more appropriately than it now fits the old.

Gallup polls provide some illustration of the change. Half a century ago, in the days of Eisenhower and Johnson, it was normal for US leaders to enjoy cross-party support from a majority of voters of the rival party. Even in Nixon's re-election year of 1972, 39% of Democratic voters gave the president a positive rating. Since then, cross-party support for the man in the White House has slumped to 22% among Republicans for Clinton and 15% among Democrats for Bush.

At the same time, US party politics has become increasingly rigid. In the past, pragmatic cooperation in Congress "across the aisle" was normal. Today it is a rarity. Republican and Democratic members of Congress now vote with their own party on 90% of all votes, a level of uniformity unimaginable in Nixon's day. "There is no dialogue," is how the former Democratic House leader Dick Gephardt puts it. "You are either in the blue team or the red team and you never wander off." As the American commentator Ronald Brownstein says in his book The Second Civil War, US politics is witnessing an unmistakeable trend towards a level of party loyalty that is parliamentary in character.

Yet no one familiar with the working of the British parliamentary system would recognise - or welcome - the polarisation that shapes US politics as applying here. It is true that British party politics is confrontational and conducted by parties with very marked tribal instincts. Yet parliamentary politics is also marked by extensive cross-party activity in select committees, cooperation between MPs over private members' legislation, and by a culture of backbench revolt that may yet wreck Gordon Brown's plans to extend terrorist detention powers beyond 28 days.

Above all, British politics is today more dominated than ever by a culture of political convergence - one reason why the blogosphere is less influential here. All our main parties have followed Tony Blair's lead by turning their backs on ideology and attempts to gratify the party base. Instead, all three now extol principled pragmatism to solve national issues and appeal to middle-England voters. Blair's fatal hubris was to ignore the political differences between Britain and America that his own pre-Iraq career had so spectacularly embodied - by committing Britain to an anti-terror strategy that was inherently and consciously partisan in American terms.

Those who hope the US primaries will produce two sharply defined partisans should therefore beware of what they wish for. At the heart of Brownstein's book is the argument that America cannot solve its problems unless it can overcome the partisanship that has wrecked its politics. The coming weeks will show whether Hillary Clinton's battle-hardened centrism or Barack Obama's claim to be a candidate who can break with the past offers the best way of addressing this overwhelming need from the Democratic side. For all our frustrations, Britain is in a much better place for putting consensus first.